1. Field of the Invention
This invention relates to a discharging and dust removing method and a discharging and dust removing apparatus for removing electric charge and dust from a working object in the form of an electric insulating member such as a plastic film, a plastic plate, a plastic card or a paper sheet or web while it is travelling.
2. Description of the Related Art
Various methods of discharging (removing electrostatic charge from) such a processing or working object as mentioned above are already known. One method of discharging a plastic film is disclosed in Japanese Patent Laid-Open Application No. Showa 63-301495. In order to allow discharging of a working object which is travelling at a high speed and eliminate reverse charging by over-discharging, the method involves two stages of discharging operations including high frequency discharging and dc discharging and also involves feedback control of the dc discharger. In particular, a travelling object in a somewhat charged condition is first discharged by high frequency corona discharging by a high frequency discharger. Then, a potential and a polarity of residual electrostatic charge of the travelling object after completion of such high speed discharging processing are detected by means of a potential detector, and the dc discharger is automatically controlled in response to the thus detected potential and polarity so that it may cause dc corona discharging, which has the opposite polarity to that of the residual charge and is to cancel the residual potential, to occur from the dc discharger to remove the residual charge by neutralization.
However, with the discharging method, since an expensive high frequency discharger must be used and a potential and a polarity of residual charge of a travelling object after high frequency discharging processing must be detected to automatically control the magnitude and the polarity of the voltage to be applied to the dc discharger by feedback control, the control system is complicated and a high cost is required for the entire discharging apparatus.
When a plastic film is fed under the guidance of a roller in a process of manufacture and working of the plastic film, since the plastic film repeats its friction with and exfoliation from the roller, charging (frictional charging) and electrostatic discharging (exfoliation discharging) are repeated. Further, where the plastic film is a film to undergo printing, the surface of the film is treated by corona discharging in order to change the quality of the same to assure a high printing performance. As a result of such frictional charging and exfoliation discharging as well as corona discharging processing, an invisible charge pattern wherein a very large number of small positive and negative charged portions having very complicated shapes are formed closely to each other at random in a mixed condition is formed on each of the opposite front and back faces of the plastic film in accordance with manners of charging and manners of discharging. FIG. 8 illustrates an example of such invisible charge pattern which was made visible by scattering toner powder (black fine particles) of the negative polarity, which is normally used with a copying machine or the like, on a surface of a plastic film immediately after exfoliation from a roller so as to cause the toner powder to directly stick to the surface of the plastic film electrostatically and then transferring the sticking toner powder image, using a copying machine, to a paper sheet to obtain the figure shown in FIG. 8. Black portions to which toner powder stuck were positively charged portions while bright portions to which no toner powder stuck were negatively charged portions, and the intensity of the black color represents the magnitude of the electrostatic potential there.
Even if it is tried to measure, using a potential measuring instrument, a charge potential of a plastic film which exhibits such a charge pattern in which small areas of positive and negative potentials are present in a complicated mixed condition, it is only possible to measure an average polarity and potential over a wide area, which depends upon the performance of the potential measuring instrument. In particular, since a small positively charged portion and a small negatively charged portion positioned in the proximity of each other exhibit a closed electric field and exhibit an electrostatically neutralized condition with each other on the surface of the film, such small portions have little influence on the measurement of the potential measuring instrument, and it cannot be avoided that the potential measuring instrument provides only a macroscopic result of measurement over a wide area.
Further, when a face of a film is discharged using a conventional discharger, ions produced by the discharger flow by a greater amount as the charge potential of the face of the film increases, but where the charge potential is low, such ions flow little. Accordingly, when small positive and negative charged portions exhibit an electrostatically neutral condition, no ions from the discharger will flow there, resulting in failure of discharging there.
However, according to conventional discharging methods including the discharging method disclosed in Japanese Patent Laid-Open Application No. Showa 63-301495 mentioned hereinabove, a polarity and a potential of charge are estimated from a result of such macroscopic measurement as described above, and charging conditions are decided uniformly in accordance with the thus estimated polarity and potential of charge (a voltage to be applied to a discharging electrode and so forth are set), and then positive and negative ions from a discharger positioned in a spaced relationship from a film are merely irradiated one-sidedly toward one face of the film. However, the opposite face of the film is left as an open face free from a grounding member or the like. Consequently, if the face of the film has such a charge pattern as described above thereon, then it has a large number of portions which have not been discharged microscopically. Consequently, even if a discharging step is performed repetitively, small uneven not-discharged portions remain to the last, resulting in deterioration of the quality of a product in which the plastic film is used as a material. For example, in the case of a product in the form of a film such as a magnetic tape wherein a magnetic material, a coating agent and so forth are to be applied to the surface of a plastic film employed as a base, it is impossible to apply such magnetic material or coating agent uniformly to the surface of the plastic film due to a discharge pattern. Or, in order to eliminate uneven not-discharged portions, a very high voltage must be used. In this instance, a discharging action of one of the positive and negative polarities is liable to become excessively strong, which may give rise to reverse charging (charging of the opposite polarity). Thus, an additional discharging step is required to remove the charge of the opposite polarity, which deteriorates the efficiency.
Such situations are not unique to those products wherein a plastic material is employed, but similarly apply to those products wherein a glass plate is employed (for example, a glass base plate for a liquid crystal display or the like).
Further, in order to remove dust or the like sticking to a face of a film in addition to discharging, also it is a common practice to jet air or irradiate an ultrasonic wave to the face of the film. However, where the film has such a complicated charge pattern as described hereinabove formed on the face thereof, dust or the like which sticks to the face of the film by a Coulomb force by charge cannot be removed uniformly.